1. Field of the Invention
The present invention relates to a method and system for acquisition of confocal STEM (scanning transmission electron microscope) images. More particularly, the invention relates to a simplified system for acquiring confocal STEM images. The invention also relates to a method using this simplified system.
2. Description of Related Art
An electron beam extracted from an electron gun is accelerated by an acceleration tube and made to hit a thin-film sample. When passing through the thin-film sample, the beam interacts with the sample. As a result, the orbit or phase may be varied or the beam may interact with the atoms in the sample, thus producing energy loss.
In confocal TEM (transmission electron microscopy), impinging electrons emitted from an electron gun are focused onto a sample by condenser lenses. Electrons transmitted through the sample are again focused at the position of an aperture by an imaging lens, the aperture being located on the upper surface of a detector that lies under the sample. The aperture permits only a part of the central portion of the focused electron beam on the sample to pass through, and the central portion is detected by a detector.
FIG. 6 shows one example of the prior art instrument. The instrument has an electron gun 1 and a condenser lens 2 for focusing the extracted electrons. A sample 10 is placed behind the condenser lens 2. A piezo actuator 3 moves the sample 10 in a direction perpendicular to the optical axis. An imaging lens 4 brings a signal including electrons transmitted through the sample 10 into focus. A pinhole-type aperture 5 is located at a focal point lying under the imaging lens 4. An image signal transmitted through the aperture 5 is detected by a detector 6. The sample is moved in three dimensions by a goniometer 9.
The instrument further includes a control imaging system personal computer (PC) 7 and an electron microscope system personal computer (PC) 8. The control imaging system PC 7 receives the output signal from the detector 6 and supplies a control signal to the piezo actuator 3. The microscope system PC 8 controls the condenser lens 2, the goniometer 9, and the imaging lens 4. The operation of the instrument constructed in this way is next described.
The electron beam emanating from the electron gun 1 is finely focused onto the sample 10 by the condenser lens 2 while being scanned in two dimensions. A signal such as electrons transmitted through the sample 10 is made to impinge on the detector 6 through the imaging lens 4 and aperture 5. If the aperture 5 is made of a pinhole-type aperture, transmitted electrons coming only from the focal position can be derived.
The derived, transmitted electrons are detected by the detector 6. The output signal from the detector is applied to the control imaging system PC 7 and stored in a memory (not shown). At this time, the scanning of the sample is controlled by the electron microscope system PC 8 and synchronized with the output signal from the detector 6. Thus, a confocal image can be obtained.
In this prior art instrument, the optical system is fixed, and only the sample 10 is scanned with the piezo actuator 3. As a result, the imaging system does not need to have any scanning system.
Another known instrument of this type can obtain information in the depth direction of the sample by applying image computations of annular dark-field scanning transmission electron microscopy (ADF-STEM) to confocal scanning transmission electron microscopy (STEM) and deriving confocal STEM images.
A further known instrument of this type is described, for example, in JP-A-2008-84643 (paragraphs 0013-0027; FIGS. 1-3). In particular, the intensity of an electron beam transmitted through a sample is measured while making the focal position of the electron beam vary in the depth direction of the sample. The absorption of the electron beam into the sample and the distribution of scattering in the depth direction are measured.
Furthermore, a technique is known which uses a nanoactuator to permit a sample stage holding a sample thereon to be moved adjustably in the Z-axis direction (along the optical axis of an electron beam) and in the X-axis and Y-axis directions perpendicular to the Z-axis direction (see, for example, JP-A-2008-270056 (paragraphs 0019-0023; FIGS. 4-6 and 12-13).
Where the scanning transmission electron microscope (STEM) shown in FIG. 6 is used and a confocal STEM image should be obtained, a special pinhole-type aperture needs to be placed ahead of the detector 6. The beam is scanned over the sample such that it can pass through the pinhole to obtain a confocal image and that mixing of other signals is prevented. In addition, the imaging system is required to have a scanner for scanning the beam. In this case, it is essential that the scanning of the imaging system be done interlockingly with the scanning over the sample. It is difficult for any ordinary instrument to achieve this requirement. Furthermore, it is difficult to control.
In order to moderate the above-described problem, in the instrument of FIG. 6, the scanning of the electron beam is halted to cease the beam over the sample in the spot mode. Under this condition, the sample is scanned, thus dispensing with scanning of the imaging system. However, this instrument is also required to scan the sample on a sub-Angstrom scale. This creates the problem of sample drift. In addition, as the sample is moved in the x- and y-directions, the position shifts in the z-direction. It is difficult to control because the x- and y-directions are normally created by cooperation between the x- and y-directions of a goniometer.